Polyglucosamines are polysaccharides having glucose monomer units with amine functionality in the polysaccharide backbone. Typical polyglucosamines include, for example, chitin, chitosan, and polyglucosaminoglycans which are copolymers of N-acetylglucosamine and various glycan sugars, e.g. hyaluronic acid, chondroitin, heparin, keratan and dermatan.
Chitin and chitosan are commonly used polyglucosamines. Chitin is a glucosamine polysaccharide which contains nitrogen and is structurally similar to cellulose. Chitin is a principle substituent of the shells of various crustaceans such as shrimps, crabs and lobsters. It is also found in some fungi, algae, insects and yeasts. Chitin is not one polymer with a fixed stoichiometry but a class of polymers of N-acetylglucosamine with different crystal structures and degrees of deacetylation and with fairly large variability from species to species. Chitosan is a generic term for a deacetylated derivative of chitin. Generally speaking, chitosan is a water-insoluble random copolymer of beta-1,4-glucosamine and N-acetyl-beta-1,4-glucosamine. Typically, the degree of deacetylation in the chitosan is about 70-100 percent, although deactylation values as low as 50% have been produced commercially.
Both chitin and chitosan are insoluble in water, dilute aqueous bases and most organic solvents. However, unlike chitin, chitosan is soluble in dilute aqueous acids, e.g., carboxylic acids, as the chitosan salts. Solubility in dilute aqueous acid is therefore a simple way to distinguish chitin from chitosan.
Chitosan is unique in that it is a polysaccharide which contains primary amine groups. Chitosan and its derivatives are therefore often used as materials in metal recovery, ion exchange resins, surgical dressings and sutures, ocular bandages and lenses, and other applications in the biomedical field. Chitosan forms water-soluble salts with many organic and inorganic acids and these chitosan salt derivatives are also often used in biomedical applications.
Although polyglucosamine salts such as, for example, chitosan salts have been found to be very useful, such salts can have functional drawbacks when the pH of the system in which they are employed rises above the isoelectric point of the polyglucosamine. At this pH, (typically at pH greater than 7.0), the salt becomes the free amine and consequently water-insoluble.
In order to circumvent the difficulties associated with the water-insolubility of polyglucosamines, the polyglucosamines can be derivatized with a variety of hydrophilic electrophiles to disrupt the secondary crystal structure of the polyglucosamines and allow the polymer to dissolve more easily into aqueous solutions. Some of the known reagents used to make such derivatives of chitosan, include for example, ethylene and propylene oxide, quaternary ammonium reagents, monochloroacetic acid and various anhydrides. The preparation of some of these derivatives can require the use of special equipment to handle high vapor pressure materials, such as ethylene oxide, highly corrosive materials, such as strong acids and bases, the isolation and control of undesirable reactants, solvents and by-products, such as alkylene glycols, toluene, monochloroacetic acid and anhydrides.
In view of the difficulties associated with preparing certain polyglucosamine derivatives, such as, for example, the chitosan derivatives described above, new processes are desired for preparing such polyglucosamine derivatives which can utilize conventional equipment and less toxic reactants.